Capacitor charging circuitry is used to charge capacitive loads, for example, in conventional photoflash systems. In conventional capacitor charging circuitry, a power switch is turned on and off to control the delivery of power from a power source to the capacitive load. Under varying load conditions or output voltage requirements, the output voltage is monitored and switching is adjusted to meet the output voltage and load requirements.
One example of conventional capacitor charging circuitry 10 is shown in FIG. 1. In this circuitry 10, power is delivered to capacitor (Cout) 12 via transformer 14. When a power switch 16 is activated, current flows into a primary coil of the transformer 14. When the power switch 16 is de-activated, the energy stored in the transformer 14 is transferred to the capacitor 12. The output voltage Vout is monitored through the secondary coil of the transformer 14 via resistive divider 20 (R1 and R2). One disadvantage of this method of monitoring Vout is the loss of capacitor energy due to the leakage current flowing through the resistors R1 and R2.
The power switch 16 is activated and deactivated by a latch 18 coupled to the output of comparators 24, 26, which controls the activation of the power switch 16 in response to the primary coil current Ipri and secondary coil current Isec. Once the primary coil current Ipri exceeds a limit, the switch 16 is deactivated and the energy of the transformer 14 is transferred into capacitor 12. This method of limiting the primary coil current Ipri for current protection and charging control uses a sense resistor 30 at the primary side of the transformer 14. One disadvantage of this method of current protection and charge control is the power dissipation due to the resistance Rpri. A sense resistor 32 is also used on the secondary side of the transformer 14, and once the secondary coil current Isec drops below a limit, the switch 16 is activated to start a new charging cycle.
Accordingly, there is a need for capacitor charging circuitry and a charging control method that minimizes current leakage and power dissipation.
Although the following Detailed Description will proceed with reference being made to illustrative embodiments, many alternatives, modifications, and variations thereof will be apparent to those skilled in the art. Accordingly, it is intended that the subject matter be viewed broadly, and be defined only as set forth in the accompanying claims.